The present invention relates to a control method for jerk-limited speed control of a movable machine element of a numerically controlled industrial processing machine, such as a machine tool, production machine or a robot. In the context of this application, the term “production machine” is used in a generic sense and also includes robots which generally follow the concepts outlined here.
A mechanical system that is capable of performing oscillations (e.g., along the axes of the machine tool) is typically characterized by at least one characteristic frequency which is excited by a movement or displacement and which can be observed in the actual position value. Excitations of the various mechanisms of a machine should therefore be eliminated.
Conventional numerical controllers that control the speed of, e.g., a machine tool, production machine or a robot, typically include a jerk limitation. The goal is to reduce the load on the various axes of the machine without adversely affecting the program processing time.
A jerk limitation can slow down the buildup of the acceleration of a machine movement so as to smooth the desired value and to move the machine mechanism with the smallest possible oscillation excitation.
However, the smoothing effect of a jerk limitation depends strongly on the desired curve for the setpoint value. Measurements and theoretical studies have shown that higher-frequency acceleration and braking processes have to be performed with a lower dynamic range so as keep the excitations of the oscillatable mechanism small. This dependence applies to both short-term positioning actions as well as, for example, for controlling the entire path of the machine.
Currently, this problem is addressed by adjusting the jerk limit value and the acceleration limit value to a low value, so that even high-frequency changes in the path speed do not significantly excite the oscillations. However, setting the dynamic values very low often prevents a higher path speed and hence also a shorter program processing time, which would be otherwise desirable.
A known method for producing a speed profile that protects the machine includes a jerk limitation. In phase 1, the machine or machine element is moved with the highest acceptable acceleration. In phase 2, the acceleration remains constant so that the speed increases linearly. In phase 3, the acceleration is then decreased. In the following, the term acceleration is meant to also include a possible negative acceleration.
Accordingly, a maximum allowable path speed is reached at the end of the phase 3, which is then used for moving the machine or machine elements during phase 4. The velocities now decrease in an analog fashion during additional phases 5 to 7, so that the path speed becomes zero when a desired position is reached. For this purpose, a negative acceleration is generated in phase 5, which is kept constant in phase 6 and again reduced to zero in phase 7. The slope of the acceleration in phases 1, 3, 5, and 7 is critical for the jerk limitation of the machine. The speed curve in these phases can be described by a polynomial, whereas in the other phases the speed is linear or constant.
As discussed above, the time dependence of the jerk r(t) is composed of constant rectangular basic shapes. FIG. 1 shows the time dependence of the jerk r(t) during the time intervals tr, ta, and tv, which represent the time intervals of the jerk r(t) for an exemplary motion path of a movable machine element. The jerk r(t) is either equal to zero or is equal to its permissible maximum value r0 with a positive or negative sign: r(t) with r(t)ε{0,+r0,−r0}.
The time-dependent curve of the jerk r(t) for a movement or a displacement of the movable machine element will in the following be referred to as a jerk profile r(t).
A control method for jerk-limited speed control of a movable machine element is known from the German patent application DE 102 00 680.6. This application suggests that the transient behavior of the movable machine elements can be improved by changing or adapting the jerk profile with the help of a sin2 shape function.
The aforementioned problem is that acceleration and braking processes with a high dynamic range along the travel path x of the machine element can still cause mechanical oscillations in a certain frequency range. The dynamic properties of these processes should therefore be adapted to the characteristic properties of the machine.
It would therefore be desirable and advantageous to provide a control method for controlling a speed of a movable machine element of a numerically controlled industrial processing machine, which obviates prior art shortcomings and specifically eliminates the excitation of characteristic resonance frequencies of a machine or a movable machine element.